By Sarah Sands with Amy McKinnon
What follows is mainly about the materials needed to prepare a canvas for oil painting. Many important elements have been left out completely: stretcher bars, cross bracing, how to stretch a canvas or mount it to panel, instructions on applying the products, use of backer boards, pre-washing, etc. Some of those issues are covered in the article on cotton canvas within this issue, or can be found easily in other references. Instead we try to answer a much more fundamental and simple question: which products do you need, and how many coats should you apply?
As in all things, we began with testing. In the fall of 2010, we embarked on a large scale project to examine common recommendations for preparing canvases for oil painting, with a particular focus on the use of oil grounds. All samples were prepared on 12 oz. cotton duck canvas following manufacturer’s recommendations. For sizing we tested our Williamsburg Rabbit Skin Glue, a PVA size made for artists, as well as various combinations of GOLDEN Polymer Medium, Matte Medium, Fluid Matte Medium, GAC 100, 200 and 400. There was no sanding between coats. We then primed the canvas with Williamsburg Lead or Titanium Oil Ground, as well as traditional oil and alkyd-based grounds from other manufacturers. Our acrylic-based GOLDEN Gesso was also included, with and without sizing, since acrylic gessoes remain by far the most common and widely used grounds today. We conducted adhesion tests (ASTM 3559) of the oil grounds to the various sizes after six months and then one year. No failures have been recorded and adhesion appears to be excellent across the board. Oil penetration, often called strike-through, was determined by inspecting the back of the canvases for any signs of discoloration. Currently all test samples remain very pliable and no embrittlement or cracking was noted when either flexed or folded.
In the world of paper and fabrics, sizing refers to a wide range of materials used to seal and stiffen. While an unusual word for most, it ultimately comes from the Old French, sise, which meant to regulate or limit. While many artists continue to use hide glue for this purpose, both polyvinyl acetate (PVA) and acrylic resins have grown increasingly popular and are considered far more stable and less problematic by conservators.
Ideally a size should soak into the fabric, lessening its absorbency and flexibility by coating and locking together the individual fibers and threads. It is typically not meant to act as a ground to paint on, or to form a substantial film on top of the canvas. Sizes also play a critical role in protecting natural fibers from direct contact with drying oils, where the process of oxidization can attack the cellulose and make it brittle.
Painters wanting to use historical materials tend to prefer rabbit skin and other hide glues, which generally create a drum tight, very stiff surface. The main downside, of course, is that conservation scientists have increasingly pointed to hide glue as a primary cause of cracking since it easily absorbs and releases moisture from the air; a property known as being hygroscopic. This in turn causes the entire canvas to tighten and slacken as the Relative Humidity (RH) rises and falls. For example, when the air becomes dry and sinks below 30% RH, the hide glue will contract with considerable force, while above 70% RH, the hide glue will soften and lose strength, and by 80% will quite literally become gelatinous and fail completely. To complicate things even further, in these high humidity situations the underlying canvas is actually moving in the opposite direction, shrinking and developing such high levels of stress that it can cause the ground and paint, no longer bound to a rigid layer of glue, to actually delaminate. It is the impact of these forces moving in opposite directions, with tensions being constantly transferred from one layer to the next, that ultimately causes the extensive cracking and damage we see in so many older paintings that have grown increasingly brittle over time. (Mecklenburg, 2007a, 2007b)
For many painters and authorities, these issues spell out a death sentence for ever using this material. Yet, truth be told, as long as the relative humidity can be kept between 30-60%, rabbit skin glue continues to represent the gold standard for stiffness and strength that other sizes are compared against.
PolyVinyl Acetate (PVA)
While pH-neutral PVA adhesives have long been used in conservation and the book arts, some manufacturers now provide PVA-based sizes for use in preparing canvases. These should adequately protect fibers from the oxidizing effect of drying oils and prevent strikethrough. In our own testing, the number of required coats to achieve this was varied, with one coat clearly sufficient for a faster drying, alkyd-based ground we tested, while a minimum of two coats were required for all the slower drying, oil-based products. Always consult with the manufacturer and check for your own application to see what is optimal.
Acrylic dispersion mediums have provided some of the most durable options when it comes to sizing a canvas, and for a very long time our recommendations have remained fairly stable: apply a coat of GAC 400 to the front in order to stiffen the fabric, followed by an additional 2 coats of GAC 100 to act as an oil blocker. Many artists saw this two-step process as cumbersome, especially since GAC 400 should ideally be heat set to lessen its water sensitivity; a step most would find impractical on any larger scale. Luckily the testing we did allowed us to reexamine this issue and ultimately modify our advice.
In terms of oil blocking, two coats of GAC 100, GAC 200 and Polymer Medium can be equally effective, as well as one coat followed by a second of either Matte Medium or Fluid Matte Medium. Adhesion of different oil grounds to a wide assortment of these combinations also did quite well. That said, everything we know from research in this area, tells us that the stiffer we can make the canvas, the more stable and secure the oil painting will be in the long run. And in that regard, GAC 200 was clearly the best choice. The one caution is that is has a higher minimum film forming temperature than our other mediums and must be applied when the temperature is at least 70°F / 21°C.
For stretched canvas the three most common options artists will encounter are traditional oil grounds, faster drying alkyd ones, and of course, acrylic gessoes. Each will have its advocates and devotees, and all represent viable options.
Traditional Oil Grounds
These are usually a simple blend of white pigment with marble dust and/or barium sulfate ground in linseed oil. The marble dust and barium sulfate serve to provide tooth and modify the absorbency and sheen of the surface. In terms of pigments, Titanium White has become the most common; its opacity and high tint strength helping to mask the natural tendency of linseed oil to yellow with age. Lead White, however, is by far the most flexible and durable option, and there is reason to believe it has beneficial effects on the painting as a whole. Lead does come with significant health warnings, however, and should be handled with great care. One should never sand a lead ground as the resulting dust and airborne particles present a significant health hazard. Zinc oxide is not frequently found in oil grounds since it will form a particularly brittle film and current research shows a strong correlation between its use and increased risk of cracking and delamination later on. Because of this, we believe one should err on the side of caution and avoid its use in grounds altogether.
For Williamsburg Oil Grounds one should wait a minimum of two weeks after the last layer is fully touch dry before using. This will give time for the ground to pass through the most active period of the drying process, when the oil is still absorbing oxygen and gaining in mass. For a fuller explanation, please see our article in Just Paint Issue 25, “Weighing in on the Drying of Oils“.
Dark yellowing is a well-known phenomenon but can be very disconcerting if you are not familiar with it. It occurs when a canvas is stored in the dark or turned against the wall for a prolonged period. During this time there is a build up of unstable chromophores that create a yellow caste but are easily bleached by light. If this happens, allow the primed canvas to be exposed to sunlight or other light source and the yellowing should lessen considerably or disappear altogether.
Alkyd-based Oil Grounds
Generally thought of and marketed as a type of oil ground, these are actually oil-modified alkyd resins that dry quickly and can often be painted on in a matter of days. Like all oil grounds, there are concerns about long-term flexibility and brittleness, especially in colder temperatures. (Young, Hagen 2008)
By far the most common product currently used, acrylic gesso provides an alternative to the oil-based products reviewed above. Fast drying, permanently flexible, and with an optimal level of tooth and absorbency. Not to be confused with traditional hide glue gessoes that can only be applied to rigid supports. A minimum of three layers should be applied to prevent oil from striking through to the back of the canvas. Increasing the number of layers acts to stiffen the surface even further, and it is not uncommon to apply 4-5 layers to create a fairly stiff surface. For a fuller explanation of this, as well as an extensive look at the research showing acrylic grounds can be safely used under oils, see our article in Just Paint Issue 24, &quto;Using Oils with Acrylics”.
Never truly transparent, a so called ‘clear gesso’ allows for the natural color of the substrate to show through. While some products are marketed specifically for this purpose, artists can actually use any number of matte mediums to achieve the same aim. In fact, our Molding Paste will be quite translucent if applied in thinner layers, and our Acrylic Ground for Pastels, while having a more pronounced tooth, can also be adapted to these needs. For general needs, we recommend using Fluid Matte Medium since it is easily applied and more resistant to long term color shifts. If you plan to leave areas of the substrate and ground exposed, we highly recommend adding a UV protective varnish at the end to limit the risk of yellowing even further.
PUTTING IT ALL TOGETHER
The above grid summarizes our best recommendations when priming canvas for oil painting based on current testing.However, it is absolutely critical that GAC 200 be applied and allowed to dry in temperatures at or above 70°F / 21°C. If you cannot maintain this temperature during that time, replace GAC 200 with the standard one coat of GAC 400 followed by two coats of GAC 100, which will provide good oil blocking and still moderate stiffness. We have also included a way to create clear grounds, as these have become increasingly popular, as well as acknowledge the continuing importance of historical materials like rabbit skin glue. Finally, we want to acknowledge that the search for a truly ideal system will continue far into the foreseeable future, and that for now, at least, painting on an inflexible support still trumps all of these in terms of stability.
Mecklenburg, Marion F., 2007a, Determining the Acceptable Ranges of Relative Humidity and Temperature in Museums and Galleries, Part 1, Structural Response to Relative Humidity, http://eprints.sparaochbevara.se/165/., 1-57. (https://repository.si.edu/handle/10088/7056)
Mecklenburg, Marion F., 2007b Determining the Acceptable Ranges of Relative Humidity and Temperature in Museums and Galleries, Part 2, Structural Response to Temperature, http://eprints.sparaochbevara.se/165/., 1-29. (https://repository.si.edu/handle/10088/7055)
Young, Christina, and Eric Hagan. 2008. Cold Temperatures Effects on Modern Paints used for Priming Flexible Supports. In Preparation for Painting: The Artist’s Choice and Its Consequences, ed. Joyce H. Townsend, Tiarna Doherty, Gunnar Heydenreich, and Jacqueline Ridge, 172-179. London: Archetype